This invention is directed to improved circuitry for driving a digital display formed of passive display elements and in particular to interfacing circuitry that is capable of delivering a plurality of discrete voltage levels to a drive circuit for driving a digital display formed of passive display elements.
While digital displays in electronic instruments requiring a large number of digits, such as calculators and the like, have taken on various forms, a preferred manner of driving a large number of display digits is by multiplex driving of the respective display digits. Multiplex driving is preferred because the circuitry required to effect same is particularly suitable for being integrated into a single circuit chip, such as an LSI chip. Integration of a multiplex driving circuit into a single chip permits the size of the chip to be reduced, reduces the number of connections between the LSI chip and the display elements, and hence simplifies the bonding therebetween, thereby increasing the yield and reducing the cost of manufacturing such integrated circuit chips.
Such multiplex driving of the digital display is particularly effective in driving digital displays of the passive variety, such as those having liquid crystal display elements, in addition to digital displays formed of light emitting diodes, fluorescent display tube, and other like elements that admit of a rectifying characteristic. It is noted however, that multiplex driving of digital displays formed of passive display elements require one-half (1/2) or one-third (1/3) AC biasing mode driving, and thereby require a plurality of discrete voltage levels to be delivered to the multiplex drive circuitry. It has been found that the type of circuitry heretofore utilized to deliver discrete voltage levels to the multiplex drive circuit chip are not particularly suited to being integrated into the same circuit chip as the multiplex driving circuitry.